Conventional human-in-mouse xenograft models offer the advantage of working with human cancer cells in vivo. A disadvantage, however, is that the human cells have been maintained in culture as distinct cell lines (NCI 60 panel) for many years. This can lead to significant differences between the properties and behavior of the xenografted cells as compared to primary tumor cells. To address the need to work with primary tumor cells, in vivo models that provide spontaneous tumors in mice have been developed. See, e.g., U.S. Pat. No. 6,639,121. In these models, however, the tumor cells are mouse tumor cells. Therefore, for human cancer research, extrapolation of experimental results across species is still necessary.